AGPLv3 is a GPLv3-compatible license that falls under the "free/libre software" tag. If you are familiar with the General Public License, AGPL differs only by an extra "web service case" which requires that modified versions of Wikidot that is used to power a web service must be also downloadable and licensed under AGPS.

Here is a short explanation what you can, must and cannot do with the Wikidot code:

CAN

Download the software and use for any purpose.

Study how the program works and adapt it to your particular needs — you have full access to the source code.

Redistribute (modified or unmodified) copies of the software.

MUST

If you are distributing original software, modified software or any software that is based on it — it has to be also licensed under AGPL.

If you have modified the Wikidot software and use it to run a web service, you have to make the modified software downloadable and licensed under AGPL. (this is what makes AGPL different from GPL).

CAN NOT

Grab the software and redistribute it under a different license.

Use modified Wikidot software and not make it available for download.

Other questions

Can I buy a commercial license for Wikidot?

If you are an individual end user, there is generally no need to purchase a commercial license, provided that you respect the terms and conditions of the AGPL.

We are however preparing a commercial license for the software that does not impose the above limitations. With the commercial license the customers would be free from the obligation of publishing their changes.

We will be also preparing support plans that would accompany commercial licenses.

How can Wikidot be both open source and commercially licensed?

This is called "dual licensing". Since Wikidot Inc. held all the copyrights to the Wikidot code, or is at least permitted to relicense or distribute code that is owned by external contributors or other parties, we are free to choose the terms under which we license the code to our customers, or the open source community.

Why dual licensing?

We — like other companies — believe that dual licensing gives both developers and users the best of two worlds. While anyone is free to use the software, look at the code and even improve it, commercial licenses support the company and allow for professional maintenance and support. The open source community gets more high-quality free software at no cost, while businesses can rely on quality support from our first-hand developers. Both worlds profit from each other: The commercial licenses support both our business and the open source community, and vice versa.

Many successful open-source software use dual-licensing in exactly the same way, e.g. MySQL, Zimbra, Qt and many other.

Will the licensing terms of Wikidot be changed in the future?

We are not currently planning to do so, but it doesn't really matter. Any code that was once released under the AGPL cannot be "closed" again. The license that Wikidot Inc. has granted you with the AGPL is perpetual and cannot be taken back. Even if Wikidot Inc. wanted to change the license again, this could only affect future versions, and anybody would still be permitted to redistribute the existing Wikidot code under the terms of the AGPL.